1. Field of the Invention
The present invention relates to an assembly of optical components for a miniature digital camera, including piezoelectric actuators for moving the optical components to provide focusing, zoom, and other functions. In particular an arrangement of the components is described that reduces the overall size and facilitates assembly.
2. Brief Description of Related Developments
The components of electronic cameras require low power consumption, low weight and cost efficiency. These design criteria are challenged by the demand for optically adjustable cameras that provide autofocus, zoom optics, or both. These features require the relative movement of optical elements to provide the adjustment. The required motion is typically linear but may use a rotating motor combined with a motion-converting mechanism such as a lead-screw. The motion range is often in the order of millimeters. It is a purpose of this invention to provide a mechanism for adjusting the position of the optical elements in an electronic camera.
One component that has been used in numerous applications is a bimorph piezoelectric element, such elements are constructed of multiple layers of piezoelectric material wherein each layer is connected for independent excitation. In U.S. Pat. No. 4,291,958, a bimorph piezoelectric cantilever beam is used in combination with a magnifying lever for focusing a camera. However, the necessary stroke of such a focusing device results in a poor stiffness of the device. In electronic camera applications, space is a crucial factor. There is thus a need for simple drive elements that can operate in narrow spaces with limited mechanical support. It is a purpose of this invention to utilize a bimorph piezoelectric element to adjust the position of a lens in an optical system of a digital camera.
A camera system using a piezoelectric actuator is described in commonly owned, related U.S. Pat. No. 6,710,950, referenced above. The disclosure of this application is incorporated herein by reference. In this application, a lens element is mounted within a camera on a tubular member. The lens tube is in turn mounted on a support tube for movement along the longitudinal axis of the tubular member. The adjustment movement is provided by means of multiple bimorph piezoelectric elements, for example by three elements, spaced symmetrically around the circumference of the support tube. The piezoelectric elements are connected to and mounted on a flexible printed circuit board which may contain other electronic components associated with the lens drive system. The flexible printed circuit board is mounted on the support tube and is in turn connected to a voltage source such as a battery. The flexibility of the printed circuit board allows it to be formed to the shape of the support tube and for the piezoelectric element to be positioned in engagement with the lens tube.
The bimorph piezoelectric element used in the mechanism of the cited application is constructed of at least two layers of piezoelectric material which are independently energized to provide relative deformation between the two layers. This piezoelectric element is formed in the shape of a beam having an engagement pad extending transverse to the plane of the element from its midpoint. The beam is fixed to the circuit board close to ends or nodal positions. The beam comprises a pair of bimorph piezoelectric elements extending to either side of the engagement pad. Each of the bimorph elements have dual active layers. The differential deformation generated by energizing only one of the two layers will cause the piezoelectric elements to bend, moving the outer end of the engagement pad into contact with the movable lens tube. By altering the excitation of the piezoelectric elements, the engagement pad causes movement in an axial direction, thereby adjusting the position of the lens. A pattern of excitation is devised to provide movement in discrete steps.
In the system of the cited application, a processor is connected in the printed circuit board to provide the main control for the digital camera and is constructed to generate a drive voltage pattern in accordance with the desired movement of the lens.
The movement generated by the piezoelectric element provides a high resolution, but there are no structural features that provide a reference in order to obtain accurate repeatability. The step length provided by the piezoelectric element can vary with operational and environmental conditions. In order to obtain the precision required in some optical designs, a position sensor is used to monitor the position of the movable tubes. An optical sensor is used to view a reflecting surface which is mounted on the moveable tube. The reflecting surface consists of a gray-scale incorporated into the surface treatment of the moveable tube. This configuration will provide accurate positional monitoring of a moveable tube.
It is a purpose of this invention to provide a miniature digital camera system of the type described above in which the components are arranged to facilitate their assembly in an over all package that is smaller. It is another purpose of this invention to provide a rail system external to the lens support tubes for engagement by piezoelectric actuators.